Maximizing Power in a Photovoltaic Distributed Power System

ABSTRACT

A power harvesting system including multiple parallel-connected photovoltaic strings, each photovoltaic string includes a series-connection of photovoltaic panels. Multiple voltage-compensation circuits may be connected in series respectively with the photovoltaic strings. The voltage-compensation circuits may be configured to provide respective compensation voltages to the photovoltaic strings to maximize power harvested from the photovoltaic strings. The voltage-compensation circuits may be include respective inputs which may be connected to a source of power and respective outputs which may be connected in series with the photovoltaic strings.

BACKGROUND

1. Technical Field

The exemplary features presented relate to a photovoltaic powerharvesting system including multiple photovoltaic strings and, moreparticularly to system and method for maximizing power in eachphotovoltaic string. .

2. Description of Related Art

Reference is made to FIG. 1 which shows a photovoltaic power harvestingsystem 10 according to conventional art. A photovoltaic string 109includes a series connection of photovoltaic panels 101. Photovoltaicstrings 109 may be connected in parallel to give a parallel directcurrent (DC) power output. The parallel DC power output connects to theinput of a direct current (DC) to alternating current (AC) inverter 103.The AC power output of inverter 103 connects across an AC load 105. ACload 105 may be an AC load such as an AC motor or may be an electricalpower grid.

By way of a simplified numerical example, three strings 109 may be usedwith an inverter 103. If two strings 109 are equally irradiated suchthat each string operates with a string voltage of 600 volts (V) andstring current of 10 amperes (A); each of the two strings generates (10A·600 V) 6 kilowatts (kW). It is also assumed that the two equallyirradiated strings 109 may be operating at maximum power.

If however, one string 109 is partially shaded or if one or more panels101 is under performing, there may still be a string voltage of 600V asset by the other two equally irradiated strings 109, however, the stringcurrent in the one under performing string 109 may only be only 6amperes. The under performing string 109 is not operating at maximumpower point. For instance, it may be that the under performing string109 has a maximum power point of 550 volts for a current of 10 amperes.In this situation, the power lost by the under performing string 109 is1.9 kW (550V·10A−600V·6A). The under performing string 109, therefore,produces 3.6 kW (600V·6A). Overall power harvested from system 10 is,therefore 15.6 kW (3.6 kW+2·6 kW).

Reference is now made to FIG. 2 which shows another power harvestingsystem 20 according to conventional art, according to internationalpatent application publication WO2010002960. System 20 is directed toreduce power losses compared to the losses of system 10. Eachphotovoltaic string 109 includes a series connection of photovoltaicpanels 101. Each photovoltaic string 109 is connected in parallel to aninput of a DC-to-DC converter 205. The output of converter 205 connectsto a DC bus 211. The DC voltage generated by photovoltaic string 109 isconverted by converter 205 to the voltage of DC bus 211. Eachphotovoltaic string 109 together with the respective DC-DC converter 205forms a photovoltaic string module 207. A number of modules 207 withoutputs from respective DC-to-DC converters 205 may be connected inparallel to DC bus 211. The parallel combined outputs of modules 207 maybe also connected to an input of a direct current (DC) to alternatingcurrent (AC) inverter 103 via DC bus 211. Inverter 103 converts thecombined DC power outputs of modules 207 to an alternating current powerat an output of inverter 103. The output of inverter 103 connects to ACload 105.

Still referring to FIG. 2, using the same numerical example as in system10 (FIG. 1), three modules 207 may be used with inverter 103. Twostrings 109 may be equally irradiated such that each string of the twostrings operates with a string voltage of 600 volts and string currentof 10 amperes. Each of the two strings generates (10 amperes·600 volts)or 6 kilowatts. If the one remaining string 109 is under performing,there may be maximum power point for the under performing string 109 of550 volts and current of 10 amperes. Each DC-to-DC converter 205 may beconfigured to maximize power on each respective output to give 600 voltson DC bus 211. The two equally irradiated modules 207 each produce 6 kW(10 amperes·600 volts) and the under performing unit 207 produces 5.5 kW(10 amperes·550 volts). Giving an overall power harvested from system 20of 17.5 kW. It can be seen that system 20 offers an improvement of 1.9kW over system 10 in terms of minimized losses and increased powerharvested. The improvement has been achieved through multiple DC-DCconverters 205 which operate at wattage levels of around 6 kW. The highpower DC-DC converters 205 in a power harvesting system may add to thecost of installation and maintenance of the power harvesting system andmay present an overall decreased level of reliability for the powerharvesting system because DC-DC converters 205 operate at high wattagelevels.

The terms “monitoring”, “sensing” and “measuring” are used hereininterchangeably.

The terms “power grid” and “mains grid” are used herein interchangeablyand refer to a source of alternating current (AC) power provided by apower supply company.

The term “converter” as used herein applies to DC-to-DC converters,AC-to-DC converters, DC-to-AC inverters, buck converters, boostconverters, buck-boost converters, full-bridge converters andhalf-bridge converters or any other circuit for electrical powerconversion/inversion known in the art.

The term “DC load” as used herein applies to the DC inputs ofconverters, batteries, DC motors or DC generators.

The term “AC load” as used herein applies to the AC inputs ofconverters, transformers, AC motors or AC generators.

BRIEF SUMMARY

Various power harvesting systems may be provided including multipleparallel-connected photovoltaic strings, each photovoltaic stringincludes a series-connection of photovoltaic panels. Multiplevoltage-compensation circuits may be connected in series respectivelywith the photovoltaic strings. The voltage-compensation circuits may beconfigured to provide respective compensation voltages to thephotovoltaic strings to maximize power harvested from the photovoltaicstrings. The voltage-compensation circuits may include respective inputswhich may be connected to a source of power and respective outputs whichmay be connected in series with the photovoltaic strings. Thevoltage-compensation circuits may be an alternating current (AC) todirect current (DC) converter where the source of power is a source ofAC power, or a DC-of-DC converter where the source of power is a sourceof DC power. The source of power may be provided by the power grid.

The power harvesting system may include further, a direct current poweroutput attached to the parallel-connected photovoltaic strings. Thevoltage-compensation circuits may include source power inputs connectedto the direct current power output.

The power harvesting system may also include a direct current poweroutput attached to the parallel-connected photovoltaic strings and aninverter including a DC power input attached to the direct current poweroutput. The inverter preferably includes an AC power output. Theinverter may be configured to invert direct current power generated bythe parallel-connected photovoltaic strings to alternating current powerat the AC power output. The voltage-compensation circuits may includesource power inputs from the AC power output.

The power harvesting system may include a central controller operativelyattached to the voltage-compensation circuits. The central controllermay be adapted to control the compensation voltages by tracking maximumpower produced from all the parallel-connected photovoltaic strings. Apower sensor may be connected to the direct current power output and thecentral controller. The power sensor may be adapted to sense power inthe direct current power output and report a sensed power to the centralcontroller. The central controller may control the compensation voltagesto maximize power from all the parallel-connected photovoltaic stringsbased on the sensed power.

The voltage-compensation circuits may be optionally configured toprovide the compensation voltages in the photovoltaic strings additionalto the voltages provided by the series connected photovoltaic panels.

The power harvesting system may also include, multiple sensorsoperatively connected respectively to the voltage-compensation circuits.The sensors may be adapted to measure a circuit parameter of thephotovoltaic strings. The voltage-compensation circuits may be adaptedto provide the compensation voltages based on the at least one circuitparameter to maximize power in the photovoltaic strings. The circuitparameter may include respective currents flowing in the photovoltaicstrings. The at least one circuit parameter may include respectivevoltages of the photovoltaic strings.

According to features presented there is provided a power harvestingsystem which includes a photovoltaic string including a seriesconnection of photovoltaic panels and a voltage-compensation circuitconnected in series with the photovoltaic string. Thevoltage-compensation circuit may be configured to provide a compensationvoltage to the string to maximize power harvested from the photovoltaicstring. The voltage-compensation circuit may include an inputconnectible to a source of power and an output connectible in serieswith the photovoltaic string.

The power harvesting system may further include a direct current poweroutput attached to the photovoltaic string. The voltage-compensationcircuit includes a DC-to-DC converter having a source power inputconnected to the direct current power output. The voltage-compensationcircuit may have an AC-to-DC converter with an alternating current (AC)source input provided from an AC power source. The AC-to-DC converteralso includes a DC output which connects in series with the photovoltaicstring. A direct current power output attached to the photovoltaicstring and an inverter having a DC inverter input connected to thedirect current power output. The AC-to-DC converter may be connectibleat the AC source input to either a power grid, or an AC output of theinverter.

According to features presented there is provided a method in a powerharvesting system which includes a photovoltaic string. The photovoltaicstring may include a series-connection of photovoltaic panels. Themethod connects in series a voltage-compensation circuit within thephotovoltaic string. A circuit parameter may be monitored within thephotovoltaic string. A compensation voltage of the voltage-compensationcircuit may be configured based on the monitoring. The compensationvoltage may be added serially within the photovoltaic string, therebymaximizing the power harvested from the photovoltaic string. A DC loadmay be attached to the photovoltaic string. An input of thevoltage-compensation circuit may be connected to either a source of ACpower or a source of DC power. The circuit parameter may include acurrent produced by the photovoltaic string, a voltage across thephotovoltaic string or the power produced by the photovoltaic string.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 shows a photovoltaic power harvesting system according toconventional art.

FIG. 2 shows another photovoltaic power harvesting system according toconventional art.

FIG. 3 a shows a power harvesting system according to a feature of thepresent invention.

FIG. 3 b shows a power harvesting system according to another feature ofthe present invention.

FIG. 3 c shows more details of a voltage-compensation circuit shown inFIGS. 3 a and 3 b, according to a feature of the present invention.

FIG. 3 d shows an implementation of a voltage-compensation circuit shownin FIGS. 3 a and 3 b, according to another feature of the presentinvention.

FIG. 4 shows a method applied to the power harvesting systems shown inFIGS. 3 a and 3 b, according to a feature of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The features are described below to explain the presentinvention by referring to the figures.

Before explaining features of the invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of design and the arrangement of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other features or of being practiced or carried out invarious ways. Also, it is to be understood that the phraseology andterminology employed herein is for the purpose of description and shouldnot be regarded as limiting.

It should be noted, that although the discussion herein relatesprimarily to photovoltaic systems, the present invention may, bynon-limiting example, alternatively be configured using otherdistributed power systems including (but not limited to) wind turbines,hydro turbines, fuel cells, storage systems such as battery,super-conducting flywheel, and capacitors, and mechanical devicesincluding conventional and variable speed diesel engines, Stirlingengines, gas turbines, and micro-turbines.

By way of introduction, features of the present invention are directedtowards maximizing output power from under-performing or partiallyshaded photovoltaic strings in a power harvesting system of parallelconnected photovoltaic strings. The features may provide maximal overallpower output of the system and reduced installation and maintenance costof the system. The features may also provide increased reliability ofthe system, owing to lower power operating levels of switchingconverters added to each of the photovoltaic string compared with DC-DCconverters 205 used in conventional system 20.

Reference is now made to FIG. 3 a which shows a power harvesting system30 a according to a feature of the present invention. A number ofphotovoltaic panels 101 are connected in series to form a photovoltaicstring 109. String 109 is connected in series with avoltage-compensation circuit 307 to provide a compensated string 315. Asource voltage (V_(s)) may be input to voltage-compensation circuit 307.A number of compensated strings 315 may be connected together inparallel to give direct current (DC) power output 211. A power sensor370 operatively connected to central controller 313 measures the poweron DC output 211. DC power output 211 is connected to an input of a DCto alternating current (AC) inverter 103. Inverter 103 converts thecombined DC power output 211 of strings 315 to an alternating currentpower at an output of inverter 103. The output of inverter 103 connectsto AC load 105. A central controller 313 may be operatively attached toeach voltage-compensation circuit 307 by bi-directional control andcommunication lines as shown, by wireless communication or by power linecommunications in DC bus 211. Central controller 313 may include amicroprocessor with on-board memory and an interface which may includeanalogue to digital converters (ADCs) and digital to analogue converters(DACs).

Reference is now made to FIG. 3 b which shows a power harvesting system30 b according to another feature of the present invention. String 109is connected in series with voltage-compensation circuit 307 to providea compensated string 315. A source voltage (V_(s)) may be input tovoltage-compensation circuit 307. A number of compensated strings 315may be connected together in parallel to give direct current (DC) poweroutput 211. DC power output 211 is connected to an input of a DC toalternating current (AC) inverter 103. Inverter 103 converts thecombined DC power output 211 of strings 315 to an alternating currentpower at an output of inverter 103. The output of inverter 103 connectsto AC load 105. System 30 a is the same as system 30 b except thatsystem 30 b does not have central controller 313. Instead, monitoringand control in system 30 b is performed by each circuit 307, which mayinclude a microprocessor with on-board memory and an interface which mayinclude analogue to digital converters (ADCs) and digital to analogueconverters (DACs). Each circuit 307 is operatively attached to sensors320, 322 and 324. Sensors 320 and 322 may be adapted to sense thevoltage across photovoltaic string 109 as well as current in string 109.Alternatively, sensors 320 and 324 may be adapted to sense the voltageacross a compensated string 315 and current in string 315.Alternatively, sensors 324 and 322 may be adapted to sense the voltage(V_(c)) across a circuit 307 as well as current through the circuit 307.

Reference is now made to FIG. 3 c which shows more details ofvoltage-compensation circuit 307 shown in FIGS. 3 a and 3 b, accordingto a feature of the present invention. Voltage-compensation circuit 307may be implemented using a direct current (DC) to DC converter 307 a.DC-to-DC converter 307 a may be a buck circuit, a boost circuit, abuck+boost circuit or switched-mode power supply (SMPS). The output ofDC-to-DC converter 307 is connected in series within string 315 to addcompensation voltage (V_(c)) to string 315. The DC source voltage input(V_(s)) to DC-to DC converter 307 may be provided from the combined DCoutput of strings 315, or strings 109. Alternatively, DC source voltageinput (V_(s)) may be provided by a micro-inverter converting AC from themains grid or another independent source of DC power such as a batteryor DC generator. Circuit 307 as shown in FIG. 3 c, is a conventionalbuck-boost DC-to-DC converter circuit which has an input voltage V_(s)with an input capacitor C₁ connected in parallel across V_(s). Twoswitches may be implemented as field effect transistors (FET) withintegral diodes: a high side buck switch Q₁ and a low side buck switchQ₂ connected in series by connecting the source of Q₁ to the drain ofQ₂. The drain of Q₁ and the source of Q₂ may be connected parallelacross the input capacitor C₁. A node A is formed between switches Q₁and Q₂ to which one end of an inductor L is connected. The other end ofinductor L is connected to the boost circuit of buck-boost DC-to-DCconverter 307 at a node B. Node B connects two switches implemented asfield effect transistors (FET): a high side boost switch Q₄ and a lowside boost switch Q₃ together in series where the source of Q₄ connectsto the drain of Q₃ to form node B. The drain of Q₄ and the source of Q₃connect across an output capacitor C₂ to produce the output voltageV_(c) of buck-boost DC-to-DC converter 307.

Reference is now made to FIG. 3 d which shows an implementation ofcircuit 307 shown in FIGS. 3 a and 3 b, according to another feature ofthe present invention. Voltage compensation circuit 307 may beimplemented using an alternating current (AC) to DC inverter. The AC toDC inverter 307 b may be a type of switched mode power supply (SMPS).When voltage-compensation circuit 307 is an AC to DC converter 307 b,the DC output of the AC to DC converter is connected in series within astring 315. The AC input (V_(s)) to the AC to DC converter may beprovided from the mains grid, from the AC output of inverter 103 or byanother independent source of AC power.

Reference is now made to FIG. 4 which shows a method 400 which may beapplied to power harvesting system 30 b shown in FIG. 3 b, according toa feature of the present invention. In step 402, an output voltage(V_(c)) of circuit 307 is wired in series with a series-connection ofpanels 101 to form compensated string 315. The input voltage (V_(s)) tocircuit 307 may be from direct current (DC) output 211, the alternatingcurrent (AC) output of inverter 103 or a separate independent AC or DCelectric supply. Several compensated strings 315 outputs may then beconnected in parallel and further connected to the input of an inverter103 as shown in FIG. 3 b.

In step 404, a circuit parameter of each parallel connected string 315is monitored in the case of system 30 b. The circuit parameter may bethe current flowing in a string 315, the voltage across a string 315,the voltage of a photovoltaic string 109 and/or the voltage (V_(c))across a circuit 307. The current and voltages in a string 315 may beused to determine the power (P) in a string 315 or a photovoltaic string109 by virtue of power being equal to voltage (V) multiplied by current(I).

In decision block 406, a control algorithm stored in a circuit 307adjusts compensation voltage V_(c) to maximize output power of string315. In step 408, a compensation voltage V_(c) for strings 315 isconfigured based on the result of the control algorithm performed insteps 404 and 406. The compensation voltage V_(c) for strings 315 instep 408 may be a positive or a negative voltage polarity with respectto the voltage polarity of a string 109. In step 410, the compensationvoltage V_(c) is added to string 315. In the case of the positivevoltage for V_(c), the voltage of a string 315 may be increased in step408. In the case of the negative voltage for V_(c), the voltage ofstring 315 may be decreased in step 408.

Reference is still being made to FIG. 4. Method 400 may also be appliedto system 30 a (FIG. 3 a) which uses central controller 313. In the caseof system 30 a, in step 404 central controller monitors or calculates anet total power from system 30 a. The net total power from system 30 ais equal to the power produced by strings 109 subtracted from the poweradded by compensation circuits 307.

When the voltage (V_(s)) and hence power to the input of circuit 307 isderived from DC bus 211 or the output of inverter 103 to givecompensated voltage (V_(c)). The net total power from system 30 a may bederived directly by monitoring (step 404) power on DC bus 211.

When the voltage (V_(s)) and hence power to the input of circuit 307 isderived from an independent DC source or AC source such as a mainssupply to give compensated voltage (V_(c)). The net total power fromsystem 30 a may be derived by subtracting power monitored on DC Bus 211(step 404) from the power added by compensation circuits 307.

In decision block 406, compensation voltages V_(c) of all strings 315may be adjusted to maximize the net total power from system 30 a. Instep 408, a compensation voltage V_(c) for a string 315 is configuredbased on the result of the control algorithm performed in steps 404 and406. In step 410, the compensation voltage V_(c) is added to a string315.

During a sustained use of systems 30 a or 30 b over a period of time,the number and type of serial connected panels 101 in a string 315 maychange, some panels may become faulty and/or operate in a current bypassmode or panels may be replaced with ones that have different electricalcharacteristics. Under these circumstances, the control algorithmmaintains strings 315 at their maximum power point (MPP) by addingcompensation voltage to each string 315 to maintain maximum power fromeach string 315. When all strings 109 are found to be operating atmaximum power output level and maximum power point, no voltagecompensation V_(c) may be required and voltage compensation V_(c) addedto string 315 is at or near zero volts.

With respect to both systems 30 a and 30 b. In each iteration of thecontrol algorithm performed in steps 404 and 406, it may be possible tosubtract from all the compensation voltages (V_(c)) in each string 315,the minimum compensation voltage V_(c). Subtracting the minimumcompensation voltage V_(c), may prevent a drift in the compensationvoltages (V_(c)) going too high for no reason. Alternatively, it may bepossible to tie the compensation voltages (V_(c)) to a level that willoptimize the overall voltage of strings 315 to be optimal for the inputof inverter 103, thereby increasing the conversion efficiency ofinverter 103.

The present features with respect to method 400 and systems 30 a or 30b, may be compared to conventional system 20 (FIG. 2) by way of the samenumerical example, where three compensated strings 315 are used. It maybe assumed just for the purpose of the numerical example that the threecompensated strings 315 are compensated by circuit 307 which may be anAC to DC converter powered from the grid. Therefore, circuit 307receives and converts voltage (V_(s)) and hence power from theelectrical grid. If two strings 109 are equally irradiated such thateach string operates with a string 109 voltage of 600 volts and stringcurrent of 10 amperes, each of the two strings generates (10 amperes·600volts) 6 kilowatts. If one under-performing string 109 is partiallyshaded or if a panel 101 is removed or bypassed, there may be a stringvoltage of 550 Volts and current of 10 amperes, which means (10amperes·550 volts) 5.5 kilowatts may be generated by theunder-performing string 109. The maximum power 5.5 kilowatts may begenerated by the under-performing string 109 only if theunder-performing string 109 can be operated at maximum power point(MPP).

Voltage-compensation circuit 307 of the under-performing string 109 maybe configured (step 408) by controller 313 to add 50 volts (V_(c)) inseries with under-performing string 109 while maintaining the current of10 amperes (step 410). Adding 50 volts by use of voltage-compensationcircuit 307, maintains string 315 voltage at 600 volts also forunder-performing string 109. Increasing the voltage of the string 315,allows the one under-performing string 109 to operate at MPP and alsorequires an extra (10 amperes·50 volts) 500 Watts when compared to the 6kilowatts in each of the other two strings 315. The overall power outputof system 30 a or 30 b is 18 kilowatts, from two strings 109 providing12 kilowatts (2·6 kilowatts), the under-performing compensated string109 providing 5.5 kilowatts and the grid providing 500 watts (50volts·10 amperes) via circuit 307. The power provided from the 3 strings315 may be therefore, the same as system 20 at 17.5 kilowatts (2·6kilowatts+5.5 kilowatts).

The benefit of systems 30 compared with system 20 is that 500W-1 kWswitching converters 307 may be required compared with 6-10 kW switchingconverters used in system 20. The difference in power rating mayrepresent a huge improvement in cost and reliability of systems 30compared with system 20.

The indefinite articles “a”, “an” is used herein, such as “a string”, “avoltage-compensation circuit” have the meaning of “one or more” that is“one or more strings” or “one or more voltage-compensation circuits”.

Although selected features of the present invention have been shown anddescribed, it is to be understood the present invention is not limitedto the described features. Instead, it is to be appreciated that changesmay be made to these features without departing from the principles andspirit of the invention, the scope of which is defined by the claims andthe equivalents thereof.

1. A system comprising: a plurality of parallel-connected compensatedphotovoltaic strings, each compensated photovoltaic string including: aphotovoltaic string having a series-connection of photovoltaic cells;and a voltage-compensation circuit having a voltage output connected inseries with said photovoltaic string, wherein said voltage-compensationcircuit includes an input connected to a source of power separate frompower provided by the photovoltaic string, wherein thevoltage-compensation circuit is configured to provide at the voltageoutput an adjustable compensation voltage in series with a voltagegenerated by said photovoltaic string such that a total voltage of thecompensated photovoltaic string maintains a predetermined value. 2.(canceled)
 3. The system of claim 1, wherein each of saidvoltage-compensation circuits include one of: an alternating current(AC) to direct current (DC) converter, wherein the AC to DC converterincludes an input connected to said source of power, and wherein the ACto DC converter includes an output configured to provide the adjustablecompensation voltage; and a DC-to-DC converter, wherein the DC to DCconverter includes an input connected to said source of power, andwherein the DC to DC converter includes an output configured to providethe adjustable compensation voltage.
 4. The system of claim 1, whereinsaid source of power is provided by an AC power grid. 5-9. (canceled)10. The system of claim 1, wherein each compensated photovoltaic stringfurther comprises: adapted to measure at least one circuit parameter ofsaid photovoltaic string, wherein said voltage-compensation circuit isadapted to adjust said adjustable compensation voltage based on said atleast one measured circuit parameter.
 11. The system of claim 10,wherein said at least one measured circuit parameter includes a currentflowing in said photovoltaic string.
 12. The system of claim 10, whereinsaid at least one measured circuit parameter includes a voltage acrosssaid photovoltaic string.
 13. A system comprising: a photovoltaic stringincluding a series connection of photovoltaic cells; and avoltage-compensation circuit having a voltage output connected in serieswith said photovoltaic string to form a compensated photovoltaic string,wherein said voltage-compensation circuit includes an input connected toa source of power separate from power provided by the photovoltaic cellswithin the photovoltaic string, wherein the voltage-compensation circuitis configured to provide at the voltage output au adjustablecompensation voltage in series with a voltage generated by thephotovoltaic string such that a total voltage of the compensatedphotovoltaic string maintains a predetermined value.
 14. (canceled) 15.The system of claim 13, wherein said voltage-compensation circuitincludes a direct current (DC-to-DC converter having: a direct currentsource power input connected to said source of power; and a directcurrent output configured to provide the adjustable compensationvoltage.
 16. The system of claim 13, wherein said voltage-compensationcircuit includes: an alternating current-to-direct current converterhaving: an alternating current source input connected to said source ofpower; and a direct current output configured to provide the adjustablecompensation voltage.
 17. (canceled)
 18. A method comprising: monitoringa circuit parameter within a photovoltaic string; receiving with avoltage-compensation circuit a source of power separate from powerprovided by photovoltaic cells within the photovoltaic string;generating from the source of power, with the voltage-compensationcircuit, a compensation voltage based on said monitored circuitparameter; and adding serially said compensation voltage with aphotovoltaic voltage generated by said photovoltaic string such that atotal voltage of the compensation voltage and the photovoltaic stringvoltage maintains a predetermined value. 19-20. (canceled)
 21. Themethod of claim 18, wherein said circuit parameter includes one of acurrent produced by said photovoltaic string, a voltage across saidphotovoltaic string, and power produced by said photovoltaic string.22-23. (canceled)
 24. The method of claim 18, further comprising:adjusting the compensation voltage such that power harvested from saidphotovoltaic string is maximized.
 25. The method of claim 18, furthercomprising: generating the compensation voltage from outputs of a directcurrent (DC) to DC converter receiving the source of power at inputs ofthe DC to DC converter.
 26. The system of claim 1, wherein each voltagecompensation circuit provides its adjustable compensation voltage so asto maximize power harvested from the photovoltaic string connected tothe voltage compensation circuit.
 27. The system of claim 1, whereineach of the voltage compensation circuits is connected to the samesource of power.
 28. The system of claim 1, wherein the predeterminedvalue for each of the compensated photovoltaic strings is the same. 29.The system of claim 28, further comprising: a power converter having aninput attached to an output of the plurality of parallel-connectedcompensated strings and having an output configured to provide thesource of power to each of the voltage compensation circuits.
 30. Thesystem of claim 13, wherein the voltage compensation circuit providesthe adjustable compensation voltage so as to maximize power harvestedfrom the photovoltaic string.
 31. The system of claim 13, furthercomprising: a sensor operatively connected to said voltage compensationcircuit, wherein said sensor is adapted to measure at least one circuitparameter of said photovoltaic string; wherein said voltage-compensationcircuit is adapted to adjust said adjustable compensation voltage basedon said at least one measured circuit parameter.
 32. The system of claim31, wherein said at least one measured circuit parameter includes one ofa current flowing in said photovoltaic string and a voltage across saidphotovoltaic string.